The present invention relates to flow nozzles that are adapted to receive plastic melt from the injection unit of an injection molding machine and direct it into a mold cavity. More particularly, the present invention relates to an improved flow nozzle that includes a removable sealing member that extends from the body of the flow nozzle to prevent leakage of plastic melt around the flow nozzle during the injection process.
In the injection molding of thermoplastic materials, a system is provided to convey the plastic melt supplied by the injection unit of the molding machine to the mold cavities. A key element is the injection nozzle, which is in fluid communication with the outlet of the injection unit and mates with the primary gate location (inlet opening) of the mold. The nozzle serves to convey the plastic melt from the outlet of the injection molding machine into the mold cavity, and it generally has an associated heater for maintaining the plastic melt at the desired temperature, thereby optimizing flow properties so that it fills the entirety of the mold cavity. It is particularly important that the nozzle seal effectively in the area adjacent the mold cavity to avoid leakage of plastic melt during the high pressure of injection.
Several sealing methods are commonly used in the industry namely, (a) sealing on the body, (b) a press-fit seal ring on the nozzle tip and (b) a flexible seal ring on the nozzle tip. The method of sealing on the nozzle body has been used for numerous years by many companies. This seal design relies on radial thermal growth of the nozzle body so that it compresses against the surrounding mold surface and accomplish the sealing task. The drawback of sealing on the nozzle body is that if the sealing surface is damaged, an expensive component, i.e., the nozzle body (made of tool steel and usually heat-treated), must be replaced.
An alternative prior art design employs a front seal member that is press-fit or shrink-fit onto the nozzle tip. The seal member is relatively flexible so that it does not have to rely only on thermal growth to properly seal. That is, the ring grows radially due to the increase in temperature, but also springs radially outward due to the pressure of the plastic melt during injection. The disadvantages of this design include: (1) the component parts have to be machined to very close tolerances in order to be press-fit or shrink-fit together; (2) during the process of press/shrink-fitting the parts together, the seal can lose its required concentricity, possibly necessitating a post-assembly machining operation; (3) if the tip is heated to a very high temperature, there is the risk that the seal will slide completely off the tip especially if the tip is made from TZM material (the thermal expansion of the seal ring is greater than the expansion of the tip)xe2x80x94to minimize this possibility, a very large press/shrink-fit interference is required, placing high stresses on both the tip and the seal ring even before they put into use; and (4) if the seal needs to be replaced, the customer must replace both the tip and seal ring as they are typically sold as an assembly.
A flexible seal ring is described on U.S. Pat. No. 5,554,395. This design is based on the concept that the seal will xe2x80x9cspringxe2x80x9d radially due to the pressure of the plastic melt during injection. Basically this design uses a standard xe2x80x9cCxe2x80x9d cross section seal ring that can be purchased from any seal ring manufacturer. This type of seal ring is also press-fit onto the tip, so that assembly of the ring would likely require special tooling. The difference between earlier press-fit seals and this patented seal appears to be the shape of the seal ring. The patented seal employs a thin (0.006-0.010xe2x80x3 wall) seal, whereas earlier designs uses a much thicker wall seal.
Another method employed in the industry for sealing the nozzle involves using a retainer to seal where the tip is held inside the body by a retainer (housing) and the sealing diameter is on the retainer. The seal works basically the same way as that of sealing on the nozzle body: it relies on thermal growth and usually seals right at the mold gate.
Briefly stated, in accordance with one aspect of the present invention, a flow nozzle is provided to convey the plastic melt supplied by the injection unit of the molding machine to the mold cavities. The flow nozzle includes a nozzle body having a passageway therethrough that extends from a plastic melt inlet to an outlet. The nozzle body is designed to threadedly receive a nozzle tip that seats in the mold gate and includes the outlet that communicates with the mold cavity. A thin-wall, cylindrical seal ring includes compression portion that is received between the nozzle body and nozzle tip and is compressed to create a seal between these two elements when the tip is assembled to the nozzle body. The seal ring further includes a sealing portion that encircles the nozzle tip and has an outer diameter slightly larger than the outer diameter of the nozzle body.
The present overcomes the disadvantages of sealing on the nozzle body since it employs a separate seal. The nozzle body of the present invention does not have a sealing surface, as such, that is prone to damage. Rather, with the present invention, only the seal ring would have to be replaced if the sealing surface is damaged, allowing use of an inexpensive replacement part (the seal ring) instead of a more expensive component (the nozzle body). In addition, by having a separate seal, a material with a lower thermal conductivity can be used to minimize the thermal losses at the seal.
The present invention has a further advantage over the prior art in that the seal ring does not depend on a press-fit or shrink-fit to create a seal with the nozzle tip. Instead, the invention relies on an axial compressive load on the seal ring to generate the sealing boundary between the tip, the seal ring and the nozzle body. Of course, the prior art designs, as well as the invention, rely on radial sealing for the seal/mold gate boundary. Accordingly, the present invention does not encounter any of the above-noted problems that are associated with prior art designs, since the seal ring and the tip are separately replaceable with respect to the nozzle body.
The present invention is also an improvement over the seal described in U.S. Pat. No. 5,554,395 in that it does not have a xe2x80x9cCxe2x80x9d shape cross-section or any of the other special shapes mentioned in the patent. The present invention is a thin, long, cylindrical seal axially aligned with the nozzle body, and does not rely solely on the pressure of the plastic melt to create the desired seal. As with other prior art designs, this patented seal is forced onto the tip and consequently relies on a radial interference. In contrast, the present invention relies on axial compression to form the seal between the tip and seal ring.